When I created a pattern for my “Owl Love” art collection in Adobe Illustrator and brought it into Photoshop as a vector object (via cut-copy-paste), thin white lines appeared in the pattern. The solution to this problem is to export the file as a PSD file with “Art Sampling” selected. (If "Text Sampling" is selected, the thin white lines still appear). The following images describe the pattern creation and exporting process. I hope this helps.